Modeling of ravine sediment loading and budget in the lower Le Sueur River, Minnesota River Basin

American Geophysical Union, Fall Meeting 2009
(2009)

Patrick Belmont, Utah State University

L A Azmera

F R Miralles-Wilhelm

R Garica

A M Melesse

Abstract

This study focuses on quantifying the sediment budget of deeply incised ravines in the lower Le Sueur River watershed in southern Minnesota. In an effort to explicitly quantify the ravine sediment budget and ravine sediment loading rate to the Le Sueur River, the high-rate-gully-erosion equations developed by Torri & Borselli (2003) along with the USLE were implemented in a numerical modeling approach that is based on a time-integration of the water and sediment balance equations. The developed numerical model calculates the rates of ravine change in its morphological characteristics (length, depth, width, area, and volume) and the amount of sediment periodically flushing from the ravines. The sediment budget of the ravines is computed as the difference between the storage of sediment and the sum of sediment loads derived from the agricultural fields, ravine side walls and terraces inside the ravines. The Torri and Borselli (2003) formulation provides the basic sediment rate equations for three components of the sediment budget; ravine side walls, ravine bed and settling sediment. The rate of sediment from upstream entering the ravine and sediment leaving the ravine is calculated using the measured total suspended sediment at the ravine head and mouth. Using the spatial analyst tool of ArcGIS and ArcHydro, topographic characteristics of the study ravines was derived from the Lidar-based DEM of the area. The area of the watershed, channel length, ravine surface area, and average slope were directly calculated in ArcGIS. The USLE was selected to estimate the rate of sediment coming from the uplands. After the erosion equations were arranged, a 30 year survey and sediment data from gully growth-rate study in Iowa by Thomas et al. (2004) was used as a case study. In an attempt to justify the assumptions and simplifications of the theoretical framework, the model output was found to compare favorably with the sediment balance components found in this reference. Using the available sediment and water flow data for the Le Sueur ravines, the Lidar-based DEM of the area, along with reasonable assumptions of some parameters the model was run to quantify the sediment budget in the study ravines. The sediment measurements in the ravines during the time period of April to October 2008 were simulated with the developed model, suggesting ranges of values for the key modeling parameters. Recommendations to guide further monitoring efforts in the watershed and increased detail modeling approaches can be gleaned from this exercise.